The invention relates to a process for influencing the breaking length cross-machine profile of a running fibrous material web in a paper machine by use of a consistency controlled flowbox which discharges the stock suspension onto a wire or between two wires.
Flowboxes of this type are known, for example from:
(1) DE 35 14 554 A1 PA1 (2) U.S. Pat. No. 5,196,091 PA1 (3) DE 40 19 593 C2
Essential quality features of paper webs are the uniformity of paper thickness, the strengths in the machine and cross-machine directions, the fiber orientation and the basis weight, in each case viewed across the web width. The basis weight profile is of decisive importance.
DE 40 19 593 C2 discloses measures to make the profiles of these properties largely uniform. The paper stock suspension is fed to the flow chamber of the flowbox through a plurality of channels which are located alongside one another in parallel and are distributed uniformly over the machine width. A mixer is connected upstream of each channel by which the concentration and the throughput in the channel may be set variably. This avoids need for adjusting spindles at the lips of the outlet channel of the flowbox. The lips are kept straight by their more or less stiff construction, so that the outlet gap between the lips has a constant height, viewed across the width in the gap. In other constructions, for example, U.S. Pat. No. 5,196,091, the outlet gap is also adjusted.
The breaking length represents a measure of the breaking strength of the paper. It indicates the length of the paper strip that will break under the load of its own weight.
A breaking length can be determined in the machine direction of the paper web or else in the cross-machine direction. The so-called breaking length ratio presents a particular problem. From this breaking length, it is possible to determine the ratio R.sub.L /R.sub.Q (=breaking length ratio). Measurements have shown that the breaking length ratio, measured across the width of the paper web, is of variable size. In the central region across the web width, it is to a certain extent constant, whereas it decreases towards the edges. As a result, the breaking length ratio, represented over the width of the paper web, resembles the internal contour of a bathtub which is open upward or open downward.
A web having a large ratio of R.sub.L /R.sub.Q is able to absorb pronounced tensile forces in the machine direction.
This is of particular interest during processing in fast running printing machines. In the latter, R.sub.L should therefore be large in relation to R.sub.Q.
By contrast, in the case of so-called format papers, as well as for many packaging papers, a strength which, as far as possible, is equal in the machine direction and in the cross-machine direction is desired. If this is not present and if the cross-machine strength is low at the web edges, then tearing of the web occurs when it is loaded in the cross-machine direction.